Such a sensing apparatus is often coupled to a machine for grinding optical lenses, in order to measure the shape of the mount exactly, in three dimensions, before the lenses are ground to the exact shape of the mount.
To this end, the mount is fixed in pairs of clips present in the sensing apparatus.
A known sensing apparatus (EP-A-1 090 716) comprises a sensor mounted on a turret rotatable about a first axis perpendicular to a mean directrix surface of the mount. The sensor is also displaceable on the turret parallel to the first axis and perpendicularly to the first axis.
Before sensing takes place, the axis of rotation of the turret is brought substantially to the centre of a first mount rim using means for displacement in a plane perpendicular to the axis of rotation of the turret. Then, the turret is immobilised and the sensor is displaced relative to the turret in order to come into contact with the groove of the mount. The turret is then driven in rotation on at least one complete turn about its axis, while maintaining the sensor in contact with the bottom of the groove to follow the shape of the mount. The axis of rotation of the turret remains generally immobile during the rotation.
The sensing apparatus registers, for each angle θ of rotation of the turret, the height z of the mount, and the vector radius p, i.e. the distance which separates the contact surface of the sensor from the axis of rotation of the turret.
The dimensions of the mount must be taken with great accuracy, since any error in the acquisition of the dimensions of the mount will result in defects in the ground shape, which subsequently cause difficulties in assembling the lenses on the mount.
The sensing apparatus described in EP-A-1 090 716 is satisfactory when the mounts are rigid and are substantially in the shape of a circle.
However, the mounts of spectacles are becoming more flexible and nowadays have elongated shapes with sharp angles. In this case, the apparatus of the aforesaid type does not give complete satisfaction.
If the sensor is really perpendicular to the groove of the mount in the non-angular parts thereof, as soon as the sensor reaches a corner of the mount, it then assumes a configuration almost parallel to the local axis of the sections of the mount.
In this case, taking into account the sudden variation in local orientation of the mount when passing the corner, the sensor exerts a force on the mount sufficient to cause it to shift or to deform it locally. In this case, the taking of the dimensions of the mount is inaccurate, thereby leading to defects during grinding.
It is therefore an aim of the invention to provide a sensing apparatus which makes it possible to carry out reliably and accurately the sensing of a mount having elongate and/or angular shapes.